The present invention relates to a process for fabricating a solid-state imaging device having a scanning circuit and a photoconductive film formed in layers on a semiconductor substrate.
In order to provide enhanced light sensitivity, a solid-state imaging device of the type contemplated by the present invention is provided with an amorphous silicon film formed on a substrate for a scanning circuit of either the MOS, CCD or BBD type. The device may include a semiconductor substrate having arranged thereon, in the stated order, a scanning circuit for scanning plural pixels, a photoconductive film, and a transparent electrode layer.
One problem with this device is that defects such as voids or cracks can easily develop in those areas of the amorphous silicon layer which correspond to step portions such as the edges of an electrode on the substrate for a scanning circuit and contact holes. The areas of the photoconductive layer containing these defects have a tendency to be etched faster than the other areas during patterning for providing a separate isolation region in the photoconductive layer between pixels. In addition, a leakage current can flow in such defective areas to cause considerable deterioration of the characteristics of the device.
Another problem with the solid-state imaging device using amorphous silicon as a photoconductive material is that the sheet resistivity of amorphous silicon is slightly lower than that of the other materials, which causes a significant reduction in resolution and increases the chance of color mixing. This problem could be eliminated by increasing the sheet resistivity of the amorphous silicon film, but then the carrier mobility is decreased or the trap density is increased, producing an after-image of greater intensity.
Another approach is to isolate the pixels in the photoconductive layer, but during etching procedures for providing a separate isolation region or making holes in an interlevel insulating layer, undesired areas of the photoconductive layer may be etched, resulting in an increased chance of shorting.
A further problem arises from the relatively large thickness (0.5 to 5 microns) of each pixel in the photoconductive layer; that is, the conductivity of a transparent electrode layer covering the photoconductive layer may be interrupted at step portions in the latter.